Angiotensin converting enzyme is being studied as an enzyme in the pulmonary circulation which alters a substrate in the venous return to the lungs before it enters the systemic circulation. In this case, the enzyme is a dipeptidylhydrolase which converts angiotensin 1 to vasoactive angiotensin 2. The enzyme is being obtained and purified from rat lung subcellular particles, since it appears to exist in lung predominantly in association with membranes. Properties of the enzyme and its molecular structure are being evaluated. Techniques are being worked on to determine its precise localization in lung tissue. Studies on the kinetics of turnover of converting enzyme and possible interrelations with enzyme in plasma are being planned.